You can add items with the following code:

// Add product
var product = Product.All().First(); // Grab any product
SiteContext.Current.OrderContext.GetBasket().PurchaseOrder.AddProduct(product);

And request payment like so:

// Grab purchase order to get order total and billing currency for request
var purchaseOrder = SiteContext.Current.OrderContext.GetBasket().PurchaseOrder;

// Find a payment method by name
var paymentMethod = PaymentMethod.SingleOrDefault(x => x.Name == "Creditcard");

// Grab the service for the payment method, which is the piece that actually requests payment
var service = paymentMethod.GetPaymentMethodService();

// Create a new payment request and request payment from the service
// Will typically redirect the customer to the payment gateway for payment processing
var payment = service.RequestPayment(new PaymentRequest(purchaseOrder, paymentMethod, 
  new Money(purchaseOrder.OrderTotal.Value, purchaseOrder.BillingCurrency); 

Hope this helps.